1. Field of the Invention
The present invention relates to a hole formation method, a multilayer wiring, a semiconductor device, a display element, an image display device, a system each containing a via hole formed by the hole formation method.
2. Description of the Related Art
In recent years, more and more integration and densification have been conducted on devices such as LSI (Large Scale Integrated Circuit) used in, for example, integrated circuits, and AM-TFT (Active Matrix Thin Film Transistor) used in, for example, displays.
One typical production process of the devices is photolithography. With the photolithography using an aligner, it is possible to easily form a fine pattern of 1 μm to 10 μm. With the photolithography using a stepper, it is possible to easily form a fine pattern of 1 μm or less.
However, the photolithography requires expensive devices, a large number of steps, and time and effort to treat waste liquids, which is problematic.
Meanwhile, as a simple process other than such photolithography, a fine pattern formation technology at low cost has attracted attention. Among these, printing methods such as an inkjet method and a screen printing method are expected as fine pattern formation techniques which have a small number of steps, exhibit high use efficiency of materials, can be performed with inexpensive devices and at low cost, and give less environmental loads.
In particular, techniques of fabricating AM-TFTs using printing methods have rapidly advanced, and the following techniques have reported, for example: a technique of fabricating an organic TFT of 76 ppi using a printing method (reverse printing) (see “T. Okubo, et al.: IDW '07, (2007), pp. 463-464.”), a technique of fabricating an organic TFT of 200 ppi using a micro-contact printing method (see “K. Yase, et al.: IDW '09, (2009), pp. 717-720.”), and a technique of fabricating an organic TFT of 200 ppi by combining an inkjet method with surface energy control by UV irradiation (see “K. Suzuki, et al.: IDW '09, (2009), pp. 1581-1584.”).
As one technique of forming a hole (via hole) using the above printing method, there has been proposed a hole formation technique in which a solvent capable of dissolving an insulating film is locally dropped on the insulating film using an inkjet method (see Japanese Patent Application Laid-Open (JP-A) No. 2003-518755).
However, in the above technique, the insulating film dissolved by the solvent problematically remains as residues. Furthermore, in order to form a fine hole, it is necessary to adjust the amount of each liquid droplet ejected by the inkjet method to several tens pico liters or less. The evaporation rate at ambient temperature greatly affects the formation of the liquid droplets, and thus the hole formation process is decreased in reproducibility, leading to a drop in yield.
Also, there has been proposed a hole formation technique in which liquid droplets of a liquid repellent material are applied by an inkjet method to a region where a hole is to be formed to thereby form a liquid repellent region, and a solution of an insulating film material is applied to the other region than the liquid repellent region (see JP-A No. 2009-21552).
However, in the above technique, it is necessary to apply the solution of an insulating film material to the other region than the liquid repellent region by the inkjet method, leading to a drop in throughput and uniformity of film thickness.
Moreover, there has been proposed a hole formation technique in which a hole (contact hole) of about 20 μm to about 45 μm is formed by screen printing where printing is performed in line pattern, and then printing is performed in dot pattern (discontinuous pattern) on the line pattern (see JP-A No. 2007-95783).
However, in the above technique, it is difficult to control liquid running, resulting in large variation in diameter of holes and hence being lack of reproducibility of the hole formation process.
In addition to the above-described problems, the hole formation by the printing method generally forms an interlayer insulating film having a thickness on the order of micrometers. As a result, the aspect ratio of the resultant film becomes high to make it difficult to form a fine pattern.
As described above, the hole formation method by the printing method has not yet established a process exhibiting satisfactory resolution and reproducibility.
A laser drill method has been proposed as a hole formation technique other than the photolithography and the printing method. With this laser drill method, laser light such as CO2 laser or YAG laser can be used to form a hole (opening) having a maximum diameter of about 20 μm in multilayer wirings such as printed wiring board.
For example, there has been proposed a technique of forming a fine pattern by applying laser to abrade or evaporate the laser-applied region, or by applying laser and removing the applied region through development (see JP-A No. 2005-50558).
However, the above technique poses a problem that a laser device is expensive. In addition, this technique poses another problem that when the laser-applied region is abraded or evaporated, its process margin is small which can form a hole only in the interlayer insulating film while keeping the shape of the lower electrode in a thin film such as AM-TFT.
In view of the above, demand has arisen for easy formation of a fine hole (for example, a via hole) with high reproducibility, high efficiency and low cost.